1. Field of the Invention
The present invention relates to a semiconductor manufacturing apparatus that performs a treatment for a substrate, such as a semiconductor wafer, with the use of a fluid of which flow volume is adjusted by a massflow controller.
2. Background Art
A semiconductor manufacturing process conventionally includes a step in which a substrate is treated with a predetermined gas or liquid. As a treatment step with a gas, there are, for example, a film deposition step with a film deposition gas, an oxidation step with an oxidation gas, and an etching step with an etching gas. As a treatment step with a liquid, there are., for example, a step of supplying a resist liquid to a substrate, and a step of applying on a substrate a chemical liquid containing a precursor of an insulation film.
Recently, in order to cope with a finer pattern of a semiconductor device, a thinner film thickness is required. Thus, a flow volume of a gas or liquid to be supplied should be accurately controlled, and a massflow controller is used therefor.
In a massflow controller, a fluid flowing through a narrow tube draws a heat from a heating resistance wire according to a flow volume of the fluid. That is, the massflow controller detects a flow volume of the fluid, on the basis of a change in a resistance value of the heating resistance wire depending on a flow volume of the fluid. The massflow controller includes: a flow volume detecting unit; a comparing unit that compares an output voltage (detected voltage corresponding to a flow volume) outputted from the flow volume detecting unit, with a set voltage which is set based on a set flow volume; and a flow volume adjusting valve that is operated based on a comparison output which is outputted from the comparing unit.
However, while the massflow controller is used, it sometimes occurs that an actual flow volume deviates from the set flow volume. For example, even if an actual flow volume is zero, there may be a case in which a voltage value outputted from the flow volume detecting unit is not zero, which is an error.
In addition to the zero point shift, the deviation of an actual flow volume from a set flow volume is caused by a shift of a change ratio (inclination), i.e., span, of an output voltage relative to a flow volume. One of the factors of the span shift is as follows. In a heating resistance wire on an upstream side and a heating resistance wire on a downstream side, both serving as sensors included in a bridge circuit, a temperature change amount relative to a change in flow volume, i.e., a change amount of an output voltage relative thereto is shifted from an initially calibrated one. This may be caused by, for example, a difference between an environmental temperature at the time of shipment by a manufacturer and an environmental temperature on a user's side; deterioration with elapse of the time of a coating material of a coil-like heating resistance wire (sensor); separation of a coating material from a heating resistance wire; slack of a coil of a heating resistance wire; malfunction of a circuit part; variation of a power supply voltage; and pollution (caused by corrosion or adhesion of product) of a pipeline wrapped with a sensor. As for a flow volume capable of being set in a massflow controller, a treatment is more susceptible to a flow volume error ratio of a smaller flow volume than a flow volume error ratio of a larger flow volume, under a condition of the same drift amount. For example, a flow volume error of a smaller flow volume has a larger influence on a thickness of a film formed on a semiconductor wafer surface.
In view of the recent higher integration and thinner film of a semiconductor device, a tolerance of a thickness of a film formed on a semiconductor wafer surface when it is manufactured has become strict. In order to manufacture a semiconductor wafer while maintaining a film thickness thereof within a tolerance, an almost largest flow volume is selected from flow volumes capable of being set in a massflow controller. The use of such a large flow volume can restrain a flow volume error to a smaller degree. For example, in a plurality of steps, when a set flow volume of a fluid must be largely changed depending on each step, one or more massflow controller(s) of a larger flow volume capacity and one or more massflow controller(s) of a smaller flow volume capacity are arranged in parallel, so as to choose (switch) a suitable massflow controller depending on the set flow volume of the fluid.
However, the use of a plurality of massflow controllers is disadvantageous in terms of costs. In addition, when an output is drifted, that is, when a fluid flow volume is zero but an output voltage therefor is not zero, the drifted output may have an adverse effect on a treatment.
On the other hand, Japanese Patent Laid-Open Publication No. 263350/1995 (especially, section 0014 and FIG. 1) describes that a measuring device, which is separated from a massflow controller, is disposed on a gas flow channel. The massflow controller is adjusted by a calibrator, based on a result measured by the measuring device.
Japanese Patent Laid-Open Publication No. 289751/1993 (especially, 9th column, lines 3 to 9) describes that the manufacturer previously, gradually changes a value of the current leading to a sensor coil of a massflow controller in an initial calibration without causing a gas to flow, so as to take out, as an unbalanced voltage of a bridge circuit, a temperature difference between the values of the current leading to the coil. By comparing the unbalanced voltage with an unbalanced voltage during use, a zero point correction amount and a span correction amount are calculated.
Since the method disclosed in Japanese Patent Laid-Open Publication No. 263350/1995 needs an additional measuring device, the method is disadvantageous in terms of costs. Further, if the measuring device itself is broken, the massflow controller cannot be handled. Moreover, a calibration performed by a calibrator is actually carried out by an operator by manually adjusting a variable resistance value. Thus, adjustment of the massflow controller at frequent intervals is troublesome.
The method disclosed in Japanese Patent Laid-Open Publication No. 289751/1993, in which an unbalanced voltage is used for adjustment, has the following disadvantages. That is, a product line is established by using a massflow controller manufactured by a certain manufacturer, which is selected out of plural massflow controllers sold by various manufacturers. However, when the massflow controller is replaced with a massflow controller of another manufacturer, the new massflow controller cannot be adjusted. In addition, there is a need for a mechanism for supplying a current to a bridge circuit while gradually changing a value of the current, which disadvantageously complicates a constitution of an apparatus.